Field of the Invention
The present invention relates to a liquid ejection head substrate configured to eject a liquid and a method for manufacturing the same.
Description of the Related Art
A liquid ejection head ejects droplets from an ejection port by heating a liquid rapidly with a heat-generating resistor disposed in a liquid ejection head substrate so as to generate bubbles. The heat-generating resistor is covered by an insulating layer configured to ensure the insulation between the heat-generating resistor and the liquid, a protective layer configured to protect the heat-generating resistor from the impact of cavitation associated with bubble generation or debubbling of the liquid, and the like. It becomes possible to heat the liquid efficiently by making these layers covering the heat-generating resistor thin. Therefore, it is desirable to make these layers thin.
However, in the configuration, in which electrode layers constituting a pair of electrodes configured to supply electric power to the heat-generating resistor are disposed on the front surface or back surface of the heat-generating resistor layer, a large height difference is generated by the pair of electrodes. In general, the electrode is composed of Al, although Al corrodes easily. Therefore, processing is difficult and the shape is not stabilized easily. Regarding the insulating layer or the protective layer disposed on the upstanding portion and the like of the height difference, the film thickness is small and the film quality is degraded compared with a film disposed on a flat portion. Consequently, if the insulating layer or the protective layer is made thin in the above-described configuration, it is difficult to protect the height difference portion generated by the pair of electrodes sufficiently, and shortages of insulation and durability to withstand impact may be caused. Also, the heat-generating resistor may be corroded by a liquid.
Japanese Patent Laid-Open No. 11-10882 proposes a configuration, in which electrodes are embedded in a heat storage layer, the surface thereof is planarized, and a heat-generating resistor layer, an insulating layer, and a protective layer are disposed on the planarized surface, as a form in which a height difference due to an electrode layer is not generated.
Meanwhile, in the case where the heat-generating resistor layer is etched in a production process of a liquid ejection head substrate, a photoresist as an example of a mask is applied to the surface of the substrate provided with the heat-generating resistor layer, and a pattern is formed by using photolithography. At this time, for example, if dry etching is performed by plasma using a gas, e.g., Cl2 or CF4, the surface of the photoresist is altered and the photoresist is not removed completely by only being dipped into an chemical solution configured to dissolve the photoresist.
At the time of bubble generation for ejecting a liquid, the heat-generating resistor reaches a high temperature such as several hundred degrees, and if the photoresist is left on the surface, a breakage may occur at an early stage. Therefore, the photoresist is removed by performing dry ashing using oxygen plasma or the like.
However, if dry ashing is performed such that the photoresist does not remain, the surface of the heat-generating resistor layer is exposed to the oxygen plasma, the surface is oxidized, a portion having a thickness of several nanometers from the surface of the heat-generating resistor layer is altered, and the resistance value of that portion increases. The thickness of the heat-generating resistor is set to be very small in order to increase the resistance value. Therefore, a profound effect is exerted by alteration of even the portion several nanometers from the surface.
The entirety of photoresist is not removed by dry asking at the same timing, the thickness of the portion altered and the degree of the alteration are different depending on the places and, therefore, there are variations in the resistance value. In addition, the durability to withstand current of the heat-generating resistor is degraded because of alteration. Consequently, if there is a portion in which the degree of alteration is great, the heat-generating resistor may break at an early stage.